Fixed some issues, improved library and PHPUnit tests

[Sebbo94BY]

• Updated README
• Move phpunit to require-dev
• Add note about installing dev dependencies in README
• Replaced spaces with tabs
• Added PHPUnit tests for MIME decoding
• Updated formatting of PHPUnit function testParsedDateTimeWithEmptyHeaderDate()
• Issue Incorrect date with time zone #209: Function to parse datetime correctly RFC2822
• Issue Error when empty headers field "from" #280: Added 'Sender' to headers and added additional if-conditions
• Issue getMail() method returns an object even for nonexistent mail ID #115: getMail() method returns an object even for nonexistent mail ID
• Issue Check error connection #273: Added connection check to example
• Issue How can I get the X-Failed-Recipients? #227: Added Failed-Recipients to IncomingMailHeader
• Issue Catch errors #140, imap_mime_header_decode may return false #246: Improved exception handling and added PHPUnit test
• Issue Catch errors #140: Added PHPUnit test for testing ConnectionException
• Issue Catch errors #140: Improved exception / error handling and improved / added PHPUnit tests
• Issue Performance improvement, my solution #154: Added ability to change the imap_search option from SE_UID to SE_FREE and added PHPUnit tests
• Issue office365 setServerEncoding US_ASCII mb_list_encodings #306: Added support for US-ASCII and added ability to disable serverEncoding for searchMailbox()
• Imported missing namespaces to avoid 'unknown class' error messages
• Issue Error in code #86: Simplified and improved one replace regex for attachment file names
• Issue Undefined property: stdClass::$host #247: Improved grabbing of fromName, fromHost, senderName and senderHost
• Issue textPlain is blank, txt is in attachment #39, Message Issue #71, multipart/related #229: Fixed body content gets incorrectly processed as attachments
• Issue Disable attachment storing #122, ignoring attachments when downloading body #150, slow!!!! #167: Added ability to skip processing of attachments to increase performance, when attachments are not required
• PR SPLIT #239:::Skip attachments #284: Added missing PHPUnit tests
• Issue Disable attachment storing #122, ignoring attachments when downloading body #150, slow!!!! #167: Lazy load message text and attachments data

[Sebbo94BY]

@barbushin please verify and merge, if it's ok from your side as well.

$ phpunit --testdox
PHPUnit 7.5.9 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.1.26-2+0~20190216175340.13+stretch~1.gbpc20626
Configuration: /home/sebastian/git/php-imap/phpunit.xml

Mailbox
 ✔ Constructor [4.25 ms]
 ✔ Constructor trims possible variables [0.37 ms]
 ✔ Server encoding has default setting and only use supported settings [0.53 ms]
 ✔ Set and get server encoding [0.06 ms]
 ✔ Imap search option has a default [0.05 ms]
 ✔ Set and get imap search option [0.10 ms]
 ✔ Get login [0.05 ms]
 ✔ Path delimiter has a default [0.39 ms]
 ✔ Path delimiter is being checked [0.10 ms]
 ✔ Set and get path delimiter [0.07 ms]
 ✔ Encoding returns correct values [0.37 ms]
 ✔ Parsed date different time zones [0.59 ms]
 ✔ Parsed date time with empty header date [0.07 ms]
 ✔ Mime encoding [0.39 ms]
 ✔ Set timeouts [1135.19 ms]
 ✔ Set connection args [0.18 ms]

Requirements
 ✔ Php imap extension is enabled [0.24 ms]

Time: 1.18 seconds, Memory: 4.00 MB

OK (17 tests, 98 assertions)

Everything, what changed (eg. ability to change the IMAP search option from SE_UID to SE_FREE) has the previous defaults, that users don't need to change their applications code, when they update php-imap.

This PR solves the in the PR description mentioned issues.

[Sebbo94BY]

@barbushin can we merge and release these changes?